In the prior art, a vehicle cooling fan control system is known which comprises a cooling fan (hereinafter, occasionally referred to as radiator fan,) to cool down a vehicle motor system as well as a condenser used in the refrigeration cycle of an air conditioner, as disclosed in Japanese Unexamined Patent Application Publication No. 2002-051588, for example.
While a vehicle key switch is turned off or the key switch is turned on but an engine is stopped, the prior art cooling fan in the vehicle cooling fan control system does not need to cool down a radiator. Therefore, it is controlled to operate or stop, or operate at a certain rotation speed to blow air for cooling down a condenser in the refrigeration cycle of the air conditioner. Further, during engine running, the cooling fan is controlled to operate or stop, or operate at a certain rotation speed based on vehicle speed, engine refrigerant temperature, turning-on and off of the air conditioner, and compressor discharge pressure.
Regarding a hybrid electric automobile which has an engine stop function during engine idling, it does not need air blowing to the radiator to cool the engine. When the air conditioner is turned off during the idling, a cooling fan motor is controlled to stop, decreasing the rotation speed.
Also, in case of an internal combustion engine vehicle in low speed driving or stopping at a traffic light, when engine coolant is low in temperature, the cooling fan is controlled under control of the air conditioner refrigeration cycle. As in the hybrid electric automobile, the turning-off of the air conditioner brings the cooling fan motor under the stop control, decreasing the rotation speed thereof.
Decreasing the rotation speed in the course of stopping, the cooling fan becomes resonant with neighboring components and generates peculiar sounds.
The peculiar sounds from the cooling fan may cause a problem of annoying an occupant in a vehicle while a vehicle interior is relatively quiet, such as during idling for a traffic light to change, at stop of engine idling of a hybrid electric automobile, or during a low speed driving due to a traffic jam.
In the hybrid electric automobile, the engine idling stop system achieves reduced gasoline consumption and reduced toxic gas emissions. Owning to its improved interior quietness, there is a problem that the peculiar sounds of the cooling fan are more conspicuous acoustically to an occupant in the hybrid electric automobile than general engine-driven vehicles.